1. Field of the Invention
The present disclosure generally relates to photodectors and more particularly to single photon detectors.
2. Description of the Related Art
A photodetector is a device that provides an electrical voltage or electrical current output signal when light is incident thereon. There are two basic types of photodetectors: linear detectors and quantum detectors. Linear detectors provide an output signal that is a linear function of the incident light intensity or average optical power. Quantum detectors provide an output signal upon detection of photons of the incident light.
A single-photon detector is a quantum detector that can detect one incident photon at a time. Commercially available single photon detectors detect photons in the visible and shorter wavelength optical regions of the electromagnetic spectrum. These commercially available detectors include silicon avalanche photodiodes (si APDs), such as part number C30954 from EG&G Optoelectronics. A typical Si APD has a responsivity of 70 A/W (amps/watt) for photons with wavelengths of 900 nm, which drops to 36 A/W for photons with wavelengths of 1064 nm. Currently available Si APDs are not sensitive enough to detect photons with wavelengths longer than 1100 nm.
Characteristics of hot-electron photodetectors that are fabricated from superconducting NbN (niobium nitride) films are discussed in K. S. Il'in, I. I. Milostnaya, A. A. Verevkin, G. N. Gol'tsman, E. M. Gershenzon, and Roman Sobolewski, “Ultimate Quantum Efficiency Of A Superconducting Hot-Electron Photodetector,” Applied Physics Letters Vol. 73, No. 26 (Dec. 18, 1998), pages 3938–3940 and in K. S. Il'in, M. Lindgren, M. Currie, A. D. Semenov, G. N. Gol'tsman, Roman Sobolewski, S. I. Chereduichenko, and E. M. Gershenzon, “Picosecond Hot-Electron Energy Relaxation in NbN Superconducting Photodetectors,” Applied Physics Letters Vol. 76, No. 19 (May 8, 2000), pages 2752–2754. Both publications are incorporated herein by reference. Some of the authors of the above mentioned articles are also inventors of this disclosure. While the first article suggests that “NbN HEPs should be able to detect single quanta of the far-infrared radiation and successfully compete as single-photon detectors with SIS-tunnel devices” (Applied Physics Letters, Vol. 73, No. 26 at p. 3940), there is no further relevant disclosure. The second article discusses the intrinsic response times of the hot-electron effect in NbN's, which applies to both linear and quantum NbN photodetectors.